A previously known torque impulse tool of this type is described in U.S. Pat. No. 5,082,066.
A problem concerned with this type of tools is that the very first delivered torque impulse tends to be powerful enough to cause a premature shut-off of the tool. This is due to the fact that in many cases the rotation speed during running down of the screw joint is very high and, accordingly, the kinetic energy of the impulse unit and the motor is very high. This kinetic energy produces a powerful first torque impulse which is strong enough to activate the retardation responsive actuation means and make the inlet valve close. The risk for a premature shut-off is particularly great when tightening so called stiff joints, i.e. screw joints having a steep torque growth characteristic per unit angle of rotation, because in such cases the first impulse is amplified by a very quick and abrupt growing torque resistance in the joint.
At screw joints with a steep torque growth characteristic, there is also a risk that the very first generated torque impulse becomes so powerful that the desired target torque level for the screw joint is passed and an undesirable torque overshoot is caused.
In the above referred U.S. Pat. No. 5,082,066, there is disclosed a speed responsive mechanism for blocking the inertia actuator at rotation speeds above a certain level. This means that the actuating means is prevented from being activated at the first torque impulse, and that the problem of having a premature power shut-off is overcome. At the impulses generated after the first one, the rotation speed and the kinetic energy of the rotating parts of the tool is considerably smaller and, consequently, the energy per impulse is much smaller too. Therefore, the blocking mechanism is deactivated and the inertia actuator is free to initiate power shut-off.
However, the problem of getting too powerful a first impulse and a subsequent undesireable torque overshoot is not overcome by this known device. There is no means provided to reduce the energy of the very first torque impulse.